The background of the invention will now be described with reference to the following drawings in which:
FIG. 1 is a schematic view of a conventional single pixel of a CRT type display.
Referring now to FIG. 1, there is shown a single pixel 1 of a normal Cathode Ray Tube (CRT) type display device. Each pixel is made up of a Red 2, Green 3 and Blue 4 phosphor dot or pixel elements. These dots are so small that when grouped together with a large number of other pixels, the light emanating from the individual dots is perceived by the viewer as a mixture of the corresponding three colors. A wide range of different colors can thus be produced by a pixel element by variation of the strength with which each phosphor dot is excited.
The display of color images in these devices is normally achieved by storing an associated value for each pixel of the display and sending this value to an intensity conversion means with the display at the requisite time. The number of different possible values stored for each pixel element of a pixel corresponds with the number of different colors which may be displayed by the display device and hence the resolution with which the device can display a given picture. With a television signal, a similar procedure is adopted of sending a pixel value to the screen corresponding to a required illumination of each particular pixel. Such procedures are well known by those skilled in the art.
By way of example, a 24 bits per pixel color display system divided into 8 bits for each of the three colors red, green and blue will be assumed. This corresponds to 2.sup.8 or 256 separate intensity levels of each red, green and blue respectively, giving 2.sup.24 different color values. A color display capable of displaying this many colors can approximate a continuous tone image to such a degree that for all practical purposes the display can be considered to be a continuous tone display.
Colors are often displayed on a computer display according to a particular model. The red, green, blue (RGB) color model is one that is in common use with CRT and color raster display devices. Other color display models include cyan, magenta, yellow (CMY) often used in color-printing devices. An example of the RGB model is the NTSC picture display standard in common use with computer displays.
As the intensity of each phosphor dot can be varied in an analogue manner, the optical center of the illumination from the phosphor dot is the centre of that dot regardless of the light intensity produced. Additionally, when multiple primary pixel elements are used to display a given color, the perceived optical center of the illumination remains substantially in the same place. Effectively, the position of a pixel is at its optical centre of illumination and, as such, all images displayed on a workstation CRT assume that the optical centers of pixel are in a regular rectangular grid.
Many display devices are unable to actually display the full range of color provided by, for example, a 24 bit input pixel. For example, a black and white raster image display can only display 2 colors, namely black and white and is known as a bi-level device. Other color display devices can only display a finite number of discrete intensity levels for each color unit. By way of further example, in a color bi-level device, such as a bilevel ferro-electric liquid crystal display (FLCD), each illumination area on the screen can be at just two intensity levels, either fully on or fully off
If the display device receives an input which has been generated on the basis that each pixel is able to display a larger number of intensity levels than can actually be displayed, then there will be an error in the color displayed, being the difference between the exact pixel value required to be displayed and the approximated value actually displayed.
Methods of generating input signals to discrete type displays have been developed to increase the number of apparent colors displayable on an discrete color display device such as a bi-level color display. The methods used are known generally as halftoning. For an explanation of the different aspects of halftoning the reader is referred to a standard textbook such as `Digital Halftoning` by Robert Ulichney, published in 1991 by MIT Press.
In order to increase the number of possible intensity levels per pixel group methods of utilizing varying size sub-pixels have been developed. For example, U.S. Pat. No. 5,124,695 (Green / Thorn EMI) discloses a pixel pattern arrangement where sub-pixels of varying size are used in relation to monochrome displays. The use of sub-pixels of varying size is also disclosed in European Patent Application 361,981 (Nakagawa et. al. / Sharp).